Dielectric filters have recently been used as high-frequency filters in mobile telephones, they particularly are required to have a reduced overall size and thickness. A flat, multi-layer dielectric filter instead of a coaxial filter is now focused. A conventional flat, multi-layer dielectric filter will be explained referring to relevant drawings.
FIG. 17 is an exploded perspective view of the conventional flat, multi-layer dielectric filter. The dielectric filter having a shown layer structure includes six dielectric substrates 1a to 1f. A shield electrode 2a is formed on the upper surface of the dielectric substrate 1b. An inter-stage coupling capacitor electrode 3 is formed on the upper surface of the dielectric substrate 1c. Resonator electrodes 4a and 4b are formed on the upper surface of the dielectric substrate 1d. Input/output coupling capacitor electrodes 5a and 5b are formed on the surface of the dielectric substrate 1e. A shield electrode 2b is formed on the upper surface of the dielectric substrate 1f. 
End electrodes 6a and 6b as grounding ports are formed on both, left and right, sides, respectively. An end electrode 7 is formed on the back side as a grounding port connected to respective open ends of the shield electrodes 2a and 2b and the resonator electrodes 4a and 4b. An end electrode 8 provided on the front side of the dielectric substrate layer structure is connected, at one end, to respective short-circuit ends of the resonator electrodes 4a and 4b, and connected, at the other end, to the shield electrodes 2a and 2b. End electrodes 9a and 9b at the left and right sides of the multi-layer dielectric substrate are connected to the input/output coupling electrodes 5a and 5b, respectively, thus operating as input/output ports.
The resonator electrodes, the inter-stage coupling capacitor electrode, and the input/output coupling capacitor electrodes of the flat, multi-layer dielectric filter are manufactured with printed patterns of conductive paste and thus are hardly have uniform thicknesses.
FIG. 18 is a cross sectional view of the dielectric substrates 1c and 1d shown in FIG. 1. As shown, the resonator electrodes 4a and 4b are thick at the center and tapered towards the edges. When the dielectric substrates are laminated, the electrodes provided by printing may be sharpened at their edge. A high-frequency current is concentrated at the edges. This reduces a Q-factor of the resonator electrode, and thus the filter has a declining performance. The conductive paste containing mainly metal powder, upon being screen-printed, may has an undulated surface due to a screen-printing mesh thus declining the performance of the filter.
The resonator electrodes, the inter-stage coupling capacitor electrode, and the input/output coupling capacitor electrodes of the flat, multi-layer type dielectric filter are provided on respective surfaces of the ceramic substrates of identical material having an identical dielectric constant. Therefore, since a current in a resonator, an essential element of the dielectric filter, concentrates at each edge of the resonator electrodes 4a and 4b, the current increase causes a conductor loss thus declining the Q factor of the resonator and the performance of the dielectric filter.